1. Field of the Invention
The present invention relates to a robot that is driven by a motor via a speed reducer, and in particular, a vertical multi-joint robot.
2. Description of the Related Arts
Nowadays, the mainstream of so-called vertical multi-joint robots for industrial uses resides in electric robots that are driven by servomotors. Generally, because servomotors have a higher rotation speed and a lower torque than the rotation speed and torque which are requested for robots, a speed reducer intervenes between a robot and a servomotor. Although a cyclone speed reducer and a harmonic speed reducer are adopted as such a speed reducer, the reduction ratio is fixed in either case, and the output of the servo motor is reduced at a constant reduction ratio.
As an exceptional example, an electric robot is disclosed by Japanese Unexamined Patent Publication No. Sho-64-51285, which is provided with means for varying the reduction ratio between a joint mechanism and the speed reducer. However, in a robot of the above-described invention, that is, in a direct teaching robot in which teaching is carried out by an operator operating the robot with its arms, the reduction ratio is changed in teaching and in playback operations, wherein the reduction ratio in the playback operation is fixed at a constant value.
However, in this type of conventional vertical multi-joint robot, a reduction ratio suited from such a maximum load is determined on the basis of a load status applied to the servomotor and is maximized with the design maximum load mass attached onto the robot, that is, a status where the rotational inertia is maximized when the arm is swung extremely outside in the case of swinging on the horizontal surface, and a status where a moment resulting from gravity with the arm horizontally shifted down in the case of swinging of a forward and backward swinging axis.
Accordingly, where the robot is used with a load mass not reaching the maximum load mass, or where the robot is used in a state where the moment due to a rotational inertia and gravity is smaller than the maximum, a problem occurs in that a sufficient operation speed cannot be obtained while the power of the servomotor becomes excessive. That is, a problem occurs in that a speed, which could be obtained if a reduction ratio responsive to a load is selected, cannot be obtained.
Therefore, it is an object of the present invention to provide a robot capable of operating at a high speed by fully utilizing the power of a servomotor.
In order to solve the above-described and other problems, a robot according to a first aspect of the invention, which is driven by a motor via a speed reduce which is made into a speed-varying reducer whose reduction ratio is varied during a playback operation of the above-described robot. The robot according to a second aspect of the invention varies the reduction ratio of the above-described speed reducer in response to the degree of an angle of the arm of the robot. The robot according to the third aspect of the invention varies the reduction ratio of the above-described speed reducer in response to the size of a load mass attached to the tip end of the arm of the above-described robot. The robot according to the fourth aspect of the invention varies the reduction ratio of the above-described speed reducer in response to the intensity of a rotational inertia around the drive shaft of the above-described robot. Also, a method for controlling a robot, which is driven by a motor via a variable speed reducer whose reduction ratio is variable during a playback motion, according to the sixth aspect of the invention comprises the steps of teaching the robot appointed operation courses; acquiring fluctuations in a rotational inertia around the drive shaft of the above-described robot and in an angle of the above-described robot arm; and determining a schedule for varying the above above-described speed reducer by the fluctuations in a load of the above-described motor resulting from the fluctuations in the above-described rotational inertia and angle of the above-described arm. The robot according to the seventh aspect of the invention comprises the steps of teaching the above-described robot appointed operation courses by an off-line teaching device to acquire fluctuations in the rotational inertia around the drive shaft of the above-described robot and angle of the above-described robot arm by simulation brought about by the above-described off-line teaching device; and determining a schedule for varying the reduction ratio of the above-described speed reducer in compliance with the fluctuations in the load of the above-described motor resulting from the above-described rotational inertia and angle of the above-described arm.